In U.S. patent application Ser. No. 334,011 filed Feb. 20, 1973, now U.S. Pat. No. 3,838,424 assigned to the same assignee as the present application, there is described a radar system for sensing velocity of a vehicle. The radar system described therein operates upon the principle that when a radar beam illuminates the ground, part of the incident power is reflected with the distribution in space similar to the speckle patterns one observes visually when the illuminator is a laser. The radar produced speckle pattern will move as a whole if there is relative motion between the radiating source and the backscattering surface. The basic technique involves measuring the time required for the speckle pattern to traverse a predetermined distance and, from this measurement calculating the velocity of the source.
The receivers of the above radar system perform well in those situations where the source pulse duration is a large fraction of the pulse repetition period. However, where the source pulse duration is small relative to the repetition period and the beam width is large, the problem of temporal decorrelation of the spatial pattern of the backscattered signal occurs because the echo returns from the various parts of the illuminated area on the ground do not exist at the receiver simultaneously. Two effects produce this non-simultaneity, first the pulse energy travelling straight downward returns much earlier relative to the energy at the edge of the antenna beam, and second if the terrain is not flat within the antenna field of view there can be significant time separation of the returns. The significance of this non-simultaneity is that the microwave speckle pattern in space becomes decorrelated and therefore is more or less washed out. This effect will be present only at and above those altitudes where the return pulse is badly stretched due to the difference in round trip distance and hence the difference in time between the earliest and latest received components of the echo return.
Conventional pulse receivers use large bandwidths providing short time constants to preserve pulse fidelity. The solution presented herein to avoid the problem of decorrelation is to employ a special narrow band receiver in which by virtue of its long time constant, the various echo components are stretched in time sufficiently to effect overlap of the total return energy. Alternately, the effect of narrow banding can be visualized by saying that only a few of the transmitted pulse sidebands are received and therefore the system may be treated on a CW rather than a pulse basis. The solution to the problem as contemplated by the present invention is to provide a receiver band sufficiently wide to maximize signal-to-noise (S/N) ratio but not so wide as to allow decorrelation of the speckle pattern because of the aforementioned lack of simultaneity. It will also be necessary to provide automatic frequency control (AFC) so that the receiver remains tuned to the altimeter transmitter. In addition, the receiver must be blanked during the transmit time to preclude overload of the receiver and obscuring of the desired return signal.